Superconducting magnets are used in a variety of contexts, including nuclear magnetic resonance (NMR) analysis, and magnetic resonance imaging (MRI). To realize superconductivity, a magnet is maintained in a cryogenic environment at a temperature near absolute zero. Typically, the magnet includes one or more electrically conductive coils which are disposed in a cryostat and through which an electrical current circulates to create the magnetic field.
There are many ways to maintain the electrically conductive coil(s) of the superconducting magnet at cryogenic temperatures so that they remain superconducting during normal operation.
In some superconducting magnet systems (for example, so-called “cryofree systems”) the magnet is maintained in a vacuum space and is cooled by a sealed system (e.g., a cold station or cold plate) which is filled with a relatively small amount of cryogenic fluid, for example one or two liters of liquid helium, so as to transfer heat from the electrically conductive coil(s) to a cold head which is in turn cooled via a compressor.
In such systems, it is beneficial to provide within the vacuum space a getter which is maintained at a very low temperature (e.g., below 20° K) so as to absorb stray molecules that may be released into the vacuum space, as such stray molecules can become a mechanism for heat transfer. In particular, over time the getter material accumulates gas molecules that may enter into the vacuum space from very small leaks.
However, it is possible that the cold head may become non-operational, for example due to a malfunction of the compressor, or due to a loss of AC Mains power for operating the compressor, thereby shutting down refrigeration of the superconducting magnet system. Such refrigeration shut down may occur during transportation, electrical outages, or equipment failure. In these cases, superconducting magnet systems with a small thermal heat capacity at low temperatures (e.g., cryofree systems having only a small amount of liquid helium inside a sealed system) may warm up rapidly above 20° K.
Meanwhile, if the getter is allowed to heat up, then the stray molecules which have been captured by the getter may be released into the vacuum chamber or cryostat which holds the superconducting magnet. If that occurs, an expensive and time-consuming vacuum pump down of the cryostat may be required to remove the released molecules.
Accordingly, it would be desired to provide a system and method for maintaining a vacuum in a superconducting magnet system in the event of a loss of cooling of a cryogenic environment in which the superconducting magnet system is deployed.
One aspect of the present invention can provide an apparatus including: a first getter material disposed within a vacuum chamber and which is configured to absorb stray molecules within the vacuum chamber; a thermal mass disposed adjacent the first getter material and in thermal communication with the first getter material; a cold station disposed within the vacuum chamber at a height greater than a height at which the thermal mass is disposed; and a convective cooling loop connected between the thermal mass and the cold station and configured to convectively cool the thermal mass when the cold station is at a lower temperature than the thermal mass, and to substantially thermally isolate the thermal mass from the cold station when the cold station is at a higher temperature than the thermal mass.
In some embodiments, the thermal mass can comprise a thermal mass of water ice.
In some embodiments, the cold station can be a 4° K cold station.
In some embodiments, the apparatus can further include: a thermal shield disposed within the vacuum chamber dividing the vacuum chamber into an inner region and an outer region; and a plurality of first low thermal conductivity support elements which connect the thermal shield to one or more outer walls of the vacuum chamber, wherein the thermal shield is isolated from the outer walls of the vacuum chamber except for the first low thermal conductivity support elements.
In some embodiments, the apparatus can further include: an independent structure disposed within an inner region of the vacuum chamber; and a plurality of second low thermal conductivity support elements which connect the independent structure to the thermal shield, wherein the independent structure is isolated from the thermal shield except for the second low thermal conductivity support elements.
In some embodiments, the apparatus can further include a plurality of third low thermal conductivity support elements which connect the thermal mass to the independent structure, wherein the thermal mass is isolated from the independent structure except for the third low thermal conductivity support elements.
In some embodiments, the apparatus can further include a thermally reflective structure disposed within the first region between the thermal mass and the thermal shield.
In some embodiments, the first getter material can comprise an activated charcoal material.
In some embodiments, the apparatus can further include a second getter material separated and apart from the first getter material and disposed adjacent to, and in thermal communication with, the cold station.
In some embodiments, the apparatus can further include a compressor disposed outside the vacuum chamber and connected to the cold station, and configured to conduct heat from the cold station to an exterior of the vacuum chamber.
Another aspect of the present invention can provide an apparatus, including: a vacuum chamber having one or more walls enclosing an interior space of the vacuum chamber; a heat shield disposed within the vacuum chamber, the heat shield defining an inner region of the vacuum chamber within the heat shield and an outer region of the vacuum chamber disposed between the heat shield and the one or more walls of the vacuum chamber; a superconducting magnet disposed within the inner region of the vacuum chamber; a cryocooler configured to cool the superconducting magnet, the cryocooler providing at least one cold station within the inner region of the vacuum chamber; a getter material disposed within the inner region of the vacuum chamber and which is configured to absorb stray molecules within the vacuum chamber; a thermal mass disposed adjacent the getter material and in thermal communication with the getter material, wherein the thermal mass is disposed at a lower greater than a height at which at least one cold station is disposed; and a convective cooling loop connected between the thermal mass and the cold station and configured to convectively cool the thermal mass when the cold station is at a lower temperature than the thermal mass, and to substantially thermally isolate the thermal mass from the cold station is at a higher temperature than the thermal mass.
In some embodiments, the apparatus can be a magnetic resonance imaging (MRI) apparatus further comprising: a patient table configured to hold a patient; gradient coils configured to at least partially surround a portion of a patient for which the MRI apparatus generates an image; a radio frequency coil configured to apply a radio frequency signal to the portion of a patient and to alter the alignment of this magnetic field; and a scanner configured to detect changes in the magnetic field caused by the radio frequency signal.
In some embodiments, the apparatus can further include: a compressor connected to remove heat from the cryocooler; and a magnet controller configured to control energization operations for the superconducting magnet.
In some embodiments, the thermal mass can comprise a thermal mass of water ice.
In some embodiments, the apparatus can further include a plurality of first low thermal conductivity support elements which connect the thermal shield to one or more outer walls of the vacuum chamber, wherein the thermal shield is isolated from the outer walls of the vacuum chamber except for the first low thermal conductivity support elements.
In some embodiments, the apparatus can further include: an independent structure disposed within an inner region of the vacuum chamber; and a plurality of second low thermal conductivity support elements which connect the independent structure to the thermal shield, wherein the independent structure is isolated from the thermal shield except for the second low thermal conductivity support elements.
In some embodiments, the apparatus can further include a plurality of third low thermal conductivity support elements which connect the thermal mass to the independent structure, wherein the thermal mass is isolated from the independent structure except for the third low thermal conductivity support elements.
Yet another aspect of the present invention can provide a method including: providing within a vacuum chamber a thermal mass adjacent to a getter material and in thermal communication with the getter material to absorb heat from the getter material; cooling the thermal mass with a cold station disposed within the vacuum chamber at a height greater than a height at which the thermal mass is disposed, in turn cooling the getter material, wherein the cooling is performed via a convective cooling loop connected between the thermal mass and the cold station; and absorbing stray molecules within the vacuum chamber with the cooled getter material, wherein the convective cooling loop substantially thermally isolates the thermal mass from the cold station when the cold station is at a higher temperature than the thermal mass.
In some embodiments, the method can further include cooling the getter material to a temperature below 20° K.
In some embodiments, the method can further include thermally isolating the thermal mass from outer walls of the vacuum chamber by a plurality of low thermal conductivity support elements.